This invention relates to communication systems and more particularly to the generation of dialing tones.
Office automation products frequently need to generate synthetic speech to communicate with operating personnel, and additionally need to generate dialing signals for dialing through the telephone network. In the past one piece of equipment has been used to generate the dialing signals and another to effect the speech synthesis, with attendent costs.
Sixteen standard signals are used for dialing through the telephone system, each associated with one of the digits, or with one of the supplementary symbols *, #, A, B, C, or D. The characteristics of the dialing signals are given in specification RS-464, a recognized industry standard. The salient features of the specification are that each of the standard dialing signals is composed of two frequency components, one from a group of higher frequencies and one from a group of lower frequencies. In particular the frequencies associated with the signals given in Table 1.
TABLE 1 ______________________________________ dialing signal high frequency low frequency ______________________________________ 1 1209 Hz 697 Hz 2 1336 697 3 1477 697 4 1209 770 5 1336 770 6 1477 770 7 1209 852 8 1336 852 9 1477 852 0 1336 941 * 1209 941 # 1477 941 A 1633 697 B 1477 770 C 1633 852 D 1633 941 ______________________________________
The standard requires that the frequency components be within 1.5% of their nominal values given above and that the level of the high frequency component of a pair be not less than and not greater than 4 db above the level of the lower frequency component.
In general, dialing signal generators have employed resonant circuits or structures readily related to the simple oscillations called for in the signals. Speech synthesizers, in contrast, are modeled on the physiological structure for producing vocalizations and have input parameters relating in a simple way to vocalization. Synthesizers have a great range of input values and a correspondingly large range of output sounds, but it is not evident how to specify the input value to stimulate the synthesizer to produce an output with two particular frequency components, or for that matter whether it can be done at all.